Aluminum lines have long been used to connect transistors of integrated circuits. As the size of transistors has decreased, the area of the cross section of aluminum lines has also decreased.
To make aluminum lines, aluminum or an aluminum alloy is sputter deposited onto a substrate as a thin layer. A photoresist is deposited thereover and patterned, and the uncovered aluminum layer etched away.
In order to ensure good pattern definition and good linewidth integrity, the etch process must be carried out so that the aluminum lines are etched anisotropically, forming vertical or slightly tapered sidewalls so as to maintain the cross section of the aluminum lines at the predetermined width and position.
Present day devices and research contemplates design rules of as little 0.50 micron in width for aluminum lines. However, for these smaller cross section aluminum lines, in the course of further processing of the devices an effect called electromigration has been noted. When a high current density is applied to a device, the aluminum lines begin to form voids in a process described as electromigration, so much so that separation of aluminum contact pads and lines has occurred, rendering the circuit useless.
It is also known to add a small amount, e.g., 0.5-1% by weight, of copper to the aluminum to reduce the electromigration effect. The use of higher copper content aluminum alloys, containing up to as much as about 4% by weight of copper, is currently being used. However, for high copper content aluminum, after standard etch processing in a single substrate etcher to form conductive lines from an aluminum layer, it has been noted that a residue remains on the etched surface, which residue has been determined to be copper. Since copper is a highly conductive metal, if the copper residue contacts an aluminum line, a short can result.
Aluminum lines deposited onto a silicon substrate are generally made up of a series of layers; a layer of sputter deposited aluminum or aluminum alloy, and a layer of, for example, TiN, TiW or W over and/or under the aluminum. Thus the etch process to form the aluminum lines is usually a multi-step etch to etch through these layers. A silicon oxide layer to be used as a hard mask can also be present over the metal layers.
A present etch process that has been used for etching aluminum comprises reactive ion etching in a single substrate etcher with a precursor gas mixture comprising; Cl.sub.2, from 10 to 80%; BCl.sub.3, from 0 to 80%; CF.sub.4, from 0 to 50%; and nitrogen, from 0 to 50%; under etch conditions of about 10-350 millitorr pressure; 200 to 1000 Watts power for the support electrode; and temperature of about 60.degree. to 100.degree. C. This process, while effective to etch aluminum or aluminum/silicon alloys containing up to about 0.5% by weight of copper, leaves a copper residue on the substrate when higher copper content aluminum alloys are used in the single substrate processing chamber.
FIG. 1 is a photomicrograph of etched aluminum lines using the above prior art process, the aluminum containing 2% by weight of copper. The copper residue can be clearly seen on the surface of the substrate.
FIG. 2 is a photomicrograph of a cross sectional view of the same etched aluminum line wherein the copper residue is seen on the surface of the substrate.
Thus a process that will etch copper-containing aluminum alloys that maintains vertical or slightly tapered aluminum sidewalls but without leaving a copper residue on the substrate would be highly desirable.